Aldehyde and Ketone Reactions: Esterification of Aldehydes and Ketones using mCPBA (RCO3H)

Esters can be created by combining aldehydes/ketones with peroxyacids (RCO3H) such as meta-chloroperoxybenzoic acid (mCPBA). This reaction also goes by the name “Baeyer Villiger Reaction”:

This reaction essentially inserts an O atom in between the aldehyde/ketone and the residual carbon chain. The O atom will be inserted before the more substituted carbon if the starting molecule is a ketone (see first example above). A frequent “trick” question with this reaction involves a ketone that is attached to a ring structure. In this situation, the ring “expands”, inserting the oxygen into the ring as seen below:

Ring Expansion Example

The reaction mechanism is depicted below:

In the first step, the lone pair electrons from the mCPBA oxygen atom attack the ketone-carbon atom, breaking the carbon-oxygen double bond, sending the free electrons to the oxygen.

For the rest of the steps, mCPBA has been replaced by RCO3H for ease of drawing. In the second step, the newly acquired lone pair electrons on the oxygen atom attack a nearby RCO3H molecule, stripping away its proton and creating the conjugate base (RCO3-)

In the third step, the conjugate base attacks the newly bound RCO3 molecule, stripping the proton away from the oxygen atom, sending the 2 electrons from the bond back to the oxygen atom.

In the fourth step, the original ketone-oxygen atom sends a lone pair of electrons to the ketone-carbon atom, creating a double bond. This in turn breaks the carbon-carbon bond, sending the electrons (and the bond) over to the oxygen atom (this is a 1,2 shift) attached to the RCO3 moiety. This causes the oxygen-oxygen bond on the RCO3 moiety to break, resulting in a free floating carboxylic acid (RCO2-).

In the fifth step, the free floating carboxylic acid returns to deprotonate the oxygen atom, thereby sending the electrons to the oxygen atom and completing the reaction.

This reaction utilizes peroxyacids, with the most common one being mCPBA however, peroxyacids as a class are generally represented as RCO3H.